The present invention relates to energy input control systems, and in particular, to a condensate sensing and control system for preventing formation of condensation on a unit being monitored.
Examples of prior art techniques for detecting moisture content of the air, e.g., dew point, or relative humidity, and/or for controlling formation of condensate on surfaces being monitored are disclosed in U.S. Pat. Nos. 2,435,895; 2,687,035; 2,720,107; 2,733,549; 2,733,607; 2,904,,995; 2,975,638; 3,142,986; 3,293,901; 3,161,056; 3,166,928; 3,195,344; 3,195,345; 3,287,974; 3,416,356; 3,422,677; 3,460,352; 3,552,186; 3,599,862; 3,696,360; 3,859,502; and British Pat. No. 900,194. Continuously heating such components is not desirable because the heated surfaces may appear warm to the touch, and because that approach involves a substantial waste of energy. It has been recognized that it is only necessary to heat the exposed surfaces being monitored periodically to keep them sufficiently warm in view of existing conditions to prevent the formation of moisture and frost.
The necessity to selectively and intermittently control a variety of electrical loads often presents significant problems. For example, commercial refrigerated units, e.g., refrigerators and freezers, particularly commercial upright units located in retail stores, are typically enclosed with by glass doors with the products contained therein visible to the consumer.
Typically, metal framed glass doors are used in these units. From the retailers point of view, it is necessary to prevent formation of condensate on these units, not only for aesthetic reasons, but more importantly because condensate, e.g., moisture and/or frost, reduces visibility through the glass doors and, reduces sales.
To overcome this problem, a number of techniques have been utilized for heating the exposed portions of the refrigerated unit e.g., the door frame, the mullion, and/or the glass itself to preclude the formation of condensate.
A number of techniques have been developed for intermittently heating the exposed surfaces of refrigerated units in an attempt to prevent the formation of condensate and to keep the surface temperatures of the glass, the door frame, the outer frame, and the mullions just above that point at which formation of condensate commences. Some of these techniques include presetting a heater to operate intermittently, but according to the fixed cycle. Another approach is to sense the relative humidity in the room in which the unit is disposed and to turn on the heaters when the relative humidity exceeds a preselected value. However, formation of condensate on the surfaces of refrigerated units is a function not only of the relative humidity in the room, but also of the temperature in the room and of the temperature of the exposed surfaces of the units, said surface temperature being partially determined by the temperature within the refrigerated units. Sensing relative humidity alone does not provide sufficient information to minimize energy utilization while simultaneously precluding formation of moisture.
Another approach is to adjust the duty cycle of the heater manually. While this may suffice, it requires constant monitoring by store personnel since the formation of frost can vary as a function of the number of times the doors are opened and as a function of changes in ambient conditions. It is common, therefore, for such systems to be set at a level to insure prevention of frost on the unit under the worst conditions, resulting in wasted energy.
As a variation of the relative humidity sensors, there are systems which adjust the duty cycle as a function of the relative humidity-increasing the duty cycle of the heaters as relative humidity increases. Again, since the point at which condensate forms is a function of more than the relative humidity in the ambient atmosphere, such systems are often adjusted to operate with a longer duty cycle than is necessary in order to preclude formation of condensate.
One of the patents identified above, U.S. Pat No. 3,696,360, discloses an alarm for warning of impending condensation on an element being monitored. While the system disclosed in this system is designed to be responsive to the various conditions which affect formation of condensation, it is believed the circuit disclosed, which includes a sensor and a load would not provide the sensitivity or accuracy required to insure prevention formation of condensation at minimum energy levels. The sensor being in the same circuit as the load, the required safety for use in areas where the sensor is exposed to personnel is not present.
In order to properly insure against formation of condensate on the exposed surfaces of a refrigerated unit, any control system should utilize as input information all of the factors which determine the point at which condensate forms on the exposed surfaces of the unit. The factors that determine this point are the ambient temperature in the room, the ambient relative humidity and the temperature of the exposed surfaces of the unit being monitored. Any satisfactory system should be reliable, automatic, efficient, should effect operation of the heaters for the minimum amount of time necessary to prevent formation of condensate, and must be safe.